Advanced electrophoretic display panels or Electrophoretic Information Displays (EPIDs) include a plurality of parallel cathode electrodes in the form of lines and a plurality of grid electrodes in the form of lines, which grid lines are transversely disposed with respect to, and insulated from, the cathode lines. The cathode lines and the grid lines are referred to as rows and columns, and the terms can be interchanged. The above-described grid-cathode structure forms an X-Y matrix which enables one to address the display at each X-Y intersection (pixel) to cause pigment particles suspended in an electrophoretic fluid to migrate to an anode electrode structure. Such electrophoretic display panels have been the subject matter of many prior art patents and the assignee herein, namely CopyTele, Inc. of Huntington Station, New York, has developed many such electrophoretic display panels as well as operating techniques for such electrophoretic display panels.
As is well known to those of ordinary skill in the art, an image is formed in an electrophoretic display panel by applying potentials to predetermined intersections of the cathode, i.e., row, and grid, i.e., column, electrodes and to the anode electrode structure. This produces predetermined electric fields which cause the pigment particles associated with the display to move to the anode. Such display operation as well as techniques for fabricating such displays are provided in U.S. Pat. No. 4,655,897, entitled "Electrophoretic Display Panels and Associated Methods", issued on Apr. 7, 1987 and in U.S. Pat. No. 4,850,819 entitled "Electrophoretic Display Panel Apparatus and Methods Therefor", issued on Jul. 25, 1989. For example, a 8.5".times.11" electrophoretic display panel having a resolution of 200 lines per inch comprises approximately 2200 cathode or row electrodes, approximately 1700 grid or column electrodes, and an overlying anode electrode structure.
In one embodiment of an electrophoretic display panel which is described in a copending patent application entitled DUAL ANODE FLAT PANEL ELECTROPHORETIC DISPLAY, filed on May 1, 1989, Ser. No. 345,825 inventors Frank J. DiSanto and Denis A. Krusos, assigned to the assignee herein, CopyTele, Inc., now U.S. Pat. No. 5,053,763, an anode electrode structure comprises conductor strips instead of a solid, thin ITO electrode layer. In such an electrophoretic display panel which is used to display characters, characters are formed utilizing a predetermined number of such anode conductor strips in a group, the predetermined number of anode conductor strips in a character line being referred to as an anode line segment.
Thus, other EPID structures include dual anode constructions as well as those EPIDs which include mesh electrodes for improving operation and display resolution. Each display apart from its construction operates basically in three different modes. In this operation the anode electrode of the display is held at a positive voltage which typically is about 200 volts. The grid voltage is usually operated at a positive voltage, which voltage is between +2 to +5 volts at the intersection of pixels to be written. The grid voltage at the intersection of pixels which are not to be written is approximately, -10-volts. The cathode under such conditions is operated at a low voltage which changes depending upon whether a pixel location is to be written into or not. This voltage goes from ground or zero volts to a voltage between +15 to +18 volts. In this manner by changing the cathode voltage from +15 volts to ground at desired pixels one can cause pigment particles to be directed towards the anode to cause a message or display to be written.
In EPIDs that utilize a mesh electrode, which is a separate individual electrode, the mesh electrode would be held at a voltage of approximately 140 volts during the write mode. In this manner, as one can ascertain, the voltage at the anode electrode, which is about 200 volts, is greater than the voltage at the mesh electrode during the write mode. Thus, in the write mode the display indicia is generated such as display characters, a picture or other indicia.
After the display is generated one may wish to remove or erase the display. Hence there is an erase mode associated with such displays. In a typical erase mode the anode voltage is directed to a source of negative potential which is typically -300 volts. In this manner all the pigment particles at the anode are caused to move away from the anode. The grid and cathode voltages in the erase operation are the same as indicated above with the grid being between +2 to +5 volts and the cathode voltage being at a low, which is ground potential. In the erase mode, all pigment particles present at the anode are directed back towards the grid to cathode structure and hence the entire image generated during the write mode is completely erased or removed during the erase mode.
There is another mode associated with the electrophoretic display and this is designated as a hold mode. In this mode an image, which was generated during the write mode, is retained during the hold mode and can continue to be displayed for extended periods of time. The held or retained image can be employed for use in facsimile or other displays. In the hold mode the anode is again placed at a positive voltage, which is 200 volts, the grid voltage is at a low value, which is a negative value of about -10 volts, and the cathode voltage is held at the high voltage which is between +15 to +18 volts. If the electrophoretic display is of the type having a mesh electrode, then during the hold mode the mesh electrode would be at a positive potential of 140 volts as in write mode. In a similar manner, during an erase mode if the display had a mesh electrode, the mesh electrode would be held at a negative potential of -200 volts. Electrophoretic displays of various structures are operated with the above-described potential in the various modes.
Another useful feature used with an electrophoretic display is the connection of an AC voltage to the mesh electrode during periods when the display is not being operated. The application of an AC voltage serves to agitate the pigment particles and to assure that no pigment particles remain on the mesh. In this manner, one connects the mesh electrode to an AC voltage with a magnitude of 100 volts rms at, for example, the 60 Hz line frequency. Other frequencies and amplitudes can be employed as well. In this mode the anode voltage is held at a positive voltage, as for example +200 volts, with the voltage at the grid at a low, which is -10 volts, with the voltage at the cathode at the cathode high voltage, which is +15 to +18 volts. The purpose of applying the AC to the mesh is to remove the pigment particles which remain at the mesh. The AC signal has no DC component and has equal positive and negative amplitudes.
Basically, when the EPID includes a mesh electrode during the "write" mode, pigment particles from the cathode are propelled to the anode. However, pigment particles also stick or remain at the positively charged mesh even though the anode is more positive than the mesh. If an AC voltage is applied to the mesh, then these particles are removed from the mesh. This AC voltage can be applied for a short period (100 milliseconds) during the "write" mode or after the "write" mode.
Such displays are operated so that after completion of the writing of an image the display panel has the anode voltage, which is equivalent to the voltage used in the hold mode at a high value, which, for example, is +200 volts. This value remains at that level until the image on the display is removed during the erase mode, as for example, where the anode is then directed to a negative potential of -300 volts or until another image is written into the display or the display is operated in the hold mode or power is turned off completely.
Electrophoretic displays employ pigment particles which are coated with surfactants and which are present in a liquid vehicle or suspension liquid. It has been discovered that there is an eventual decomposition of chemicals which decomposition is related to the amplitude of the current through the display and the time interval over which that current is circulating or propagating. Suffice it to say that under present conditions and techniques of manufacturing, the currents circulating in such displays are extremely small and the deterioration of such a display is very slow. Extensive life tests have been performed on such displays and these factors are shown to be true. Any reduction of current, when the image is written on the display and where the image has to be held for extended periods, is advantageous. In addition, by reducing the current the average power required by the panel during such hold conditions decreases substantially.
Thus, the present invention involves a method of operating an electrophoretic display whereby the voltages applied to the electrodes during a hold mode are extremely low, thereby greatly reducing the current in the display and thereby greatly reducing the power dissipated by the display while further increasing the effective life of the display while further improving performance in general.
It has also been determined that by the reduction of such voltages during the hold mode the overall appearance of the pigment particles appear much more pleasing in that the image and texture of the pigment changes thereby giving the image a more pleasant appearance than those images produced utilizing the above-described conventional techniques.